Process for preparing dialkyl esters of dicarboxylic acids



April 13, 1948. w, GRESHAM 2,439,425

rnocass FOR PREPARING DIALKYL ns'rnas or nIcAnBoxrLIc' ACIDS FiledApril 1. 1946' r GAS r 10 *vem P12;

PUMP OOOLER Patented Apr. 13, 1948 I: PROCESS FOR PREPARING DIALKYLvESTERS OF DICARBOXYLIO ACIDS William F. .Gresham, Wilmington,

DeL, asslg nor to E, I..du Pont de Nemours & Company,'Wilmington, Del.,a corporation of Delaware Application April 1, 1946, ScrialNo. 658,742Sfllaims. (or emu-'12) This invention pertains to the synthesis ofdiesters and more particularly to the synthesis of diesters of'dibasicacids by electrolysis of monoesters of dibasic acids. In a specificembodiment the invention is directed to the synthesis of dialkyl estersof sebacic acid by the electrolysis of monoalkyl adipates.

' It has been known for many years that alkali metal salts of monoestersof dibasic acids, upon electrolysis, yield diesters of higher dibasicacids (Ann, 261, 111 (1891); Bull. Soc, Chem. [3], 29,

1038 (1903); Transactions of Electrochemical So ciety 69, 287, (1936),7'7, 459, (1940)). More recently it has been observed thatcertain'amines such, as pyridine and diethylamine maybe present duringthe electrolysis of certain monocarboxylic acids, whereby a complexmixture of products is generally obtained. All. of the aforesaid priorart processes for the preparation of esters by electrolysis ofmonoesters of dibasic acids have had numerous disadvantages. Forexample, when an aqueous medium was employed, yields of dialkyl estershaving less than about 14 carbon atoms in the chain were generally notgreater than about 30%. If a methanolic medium was employed, the alkalimetal salt of the monoalkyl ester of the dibasic acid had a relativelylow solubility in the mixture,v especially after substantialquantitiesof the diester had been formed, which gave rise to operatingdifiiculties.

An object of this invention is to provide an improved process. forthepreparation of dialkyl esters, especially. dialkyl este s ofdicarboxylic acids of'the empirical formula kcnnmcooniz in which is aninteger, preferably from 2 to 7. r A further object is to provide aprocess for the preparation of dialkyl sebacates in high yield fromreadily available starting materials. A more specific object is toprovide a commercially .i I 2 trimethylamine salt of the said ester, toelectrolysis, and thereafter separating the resultant dialkyl ester of adicarboxylic acid of the empirical formula (CHzlmQCOOH): from theelectrolysis product. In a preferred embodiment the present inventioncontemplates the electrolysis of a monomethyl adipate solution, obtainedby treating monomethyl adipate with trimethylamine in a methanolsolvent, in a plate-type cell whereby dimethylsebacate is produced inhigh yield; and the trimethylamine initially introduced into theelectrolysis mixture is recoverable almost quantitatively in unchangedform.

One ofthe surprising features of the. present invention is the discoveryof the highly specific action of trimethlyamine when employed inmethanol solution. This is more readily appreciated upon considerationof the fact that other organic solvents, such as ethanol, etc., arecompletely inoperative, or nearly so, in the practice of the invention.Moreover, it is not possible to substitute 'for trimethylamine .otherorganic compounds having salt-forming properties; for

' example, pyridine is inoperative and is attacked very rapidly underthe conditions necessary for the formation of the diesters; ammonia,dimethylainine, and the like also have been found by the applicant to beof no value in the process. In general, such compounds give rise totarry materials, and virtually none of the desired diester is obtained.A few amines, such as ethylene diamine, behave-like trimethylamine, andmay be employed in an quivalent manner. a In general, however,trimethylamine gives outstanding results, and appears to be highlyspecific in the practice of the invention. This may be due to thestability of trimethylamine towards e1ecfeasible process for themanufacture of dimethyl sebacate by the electrolysis ofmonomethyladipate. Other objects of the invention will appearhereinafter.

These objects are accomplished in accordance with the invention by theelectrolysis of monoesters of dibasic acids, preferably in methanolsolution, in the presence of controlled amounts of the trimethylaminesalt of the said monoester, as set forth in reater detail hereinafter.The invention contemplates subjecting a monoalkyl ester of a dibasicacidof the empirical formula (CHQMCOOHM, n being any integer, and thetrolytic oxidation, for it is noted that methyl groups survive reactionconditions which tend to destroy ethyl groups, etc. This is furthersupported by the'fact that monomethyl esters give better yields thanmonoethyl esters in the practice ofthe invention. Whatever the expla-,

nation, trimethylamine may be employed with the monoesters of dibasicacids in place of the alkali metal salts heretofore employed, thusavoiding the above-mentioned disadvantages which are encountered inprocesses employing the said alkali metal salts. Moreover, the use ,oftrimethylamine facilitates recovery of the dialkyl ester, for it hasbeen discovered, in accordance with the invention, that the solution.withdrawn from the electrolytic cell, as herein described, is of suchnature that, upon cooling, a separation cf' dimeth yl sebacate incrystalline formtakes place. This. of course, could not be accomplishedin processes in which the alkali metal s is are electrolyzed, for theunconverted salts, be ng relatively insoluble, are precipitatedwith,,or=in preference to, the dimethyl sebacate, upon cooling theelectrolysis products. The applicant has discovered that. even iflithium 'salts are employed, the process does not operatesatisfactorily, partly due to an excessive formation of tarryby-products. 7 i

The invention will be better understood by reference to the accompanyingdrawing. The drawing represents diagrammatically an apparatus whichconsists essentially of i a centrifugal pump which introduces thesolution of monomethyl adipate, trimethylamine, and methanol into theplate-type cell 2, through a fish-tail nozale 3, which directs the flowof liquid between the cathode 4, and the mode 5. The current is passedto and from the electrodes by means of the glass-enclosed wires 6 andThus the electrolytic cell contains the cathode and anode immersed in abath of the methanol solution 8 of solution is rapidly circulatedbetween the plates solution ,is thereafter continuously withdrawnmonomethyl adipate and trimethylamine, which f 4 1.0. ampere per squarecentimeter. The best yields are obtained at the highest controllablecurrent densities.

The apparatus employed in the practice of the invention may be made ofor lined with suitable inertmaterials such as inert metals or Pyrexglass. Since the anode tends to oxidize very rapidly during theelectrolysis, it is essential that the anode be constructed of materialswhich are very resistant to oxidation, such -as metallic platinum,platinum on Carborundum, or the like. The cathodes should also beconstructed of inert materials such as, platinum, other noble metals,copby reason of the flow from the nozzle 3. .The

through the outlet 9, into a separator ii, for

release of carbon dioxide and hydrogen through the vent i0, and thewithdrawal of liquid throughthe outlet ii, to the vessel i2. The vesseli2 is equipped with a means for introducing makeup monomethyl adipateand trimethylamine through the inlet l3. It is also equipped with anoverflow ll, whereby a portion of the reaction product. can becontinuously purged, or removed to suitable devices forrecovery oftrimethylamine and dimethyl sebacate. The mixture which accumulates inthe vessel i2 is in part continuously withdrawn through the valve i5, tothe cooler l6 and thence to the pump I, which returns recovered solutionto the electrode cell 2. Alternatively the cooler i6, may be operated ata sufllciently low temperature (e. g., 50 to 0, (2.), to cause acontinuous separation of dimethyl sebacate therein. The supernatant.liquor may be recirculated to the electrolytic cell. Insuchembodiments, it is preferable to add the make-up monomethyl adipateto the supernatant liquor after the'cooling and separating of diester.

In the practice of the invention it is generally desirable to employ asthe initial reaction mixture a solution of partially neutralizedmonoester in methanol, said solution containing about 0.1 to 0.9preferably 0.3 to 0.8 mol of trimethylamine, in,the form of monoestersalt, per mol of monoester, i. e., per mol of monoester in theneutralized plus the unneutralized form. In a preferred embodimentthe'reaction mixture is a solution of monomethyl adipate in methanolobtained by dissolving about 2 mols of monomethyl adipate per liter ofmethanol and introducing also sufilcient trimethylamine to neutralizeabout 50% of the acidic hydrogen present in the said monomethyl adipate.Alternatively, a solution of monomethyl adipate in water containingabout 2 mols of monomethyl adipate per liter, 75% neutralized withtrimethylamine, may be employed but as stated above water has a highlydeleterious effect upon the yield of desired products.

The electrolysis is generally conducted at a temperature of about 0 to55 0., preferably about 20 to 45 C. The anode current density should beat least about 0.1, and preferably about 0.2 to

per, carbon or the like. Since the electrochemical reaction, and theresistance of the cell (especially after a substantial amount of diesterhas accumulated), causes the development of heat, it is generallydesirable to employ a means for removal of heat. This may be done asshown in the diagrammatic drawing by cooling the recirculated solution.Alternative methods, such as internally cooled electrodes and the likemay be employed if desired.

It is highly desirable thatthere be a very rapid flow'of the solutionbetween the electrodes. This may be achieved by means of a nozzle whichdirects the flow of the solution rapidly between the plates. Moreover,it is preferred that the electrodes be placed closely together, with aspacing of about 2 millimeters or less, since the cell resistance, atleast in part', is a function of the distance between the electrodes.

' The invention is illustrated further by means of the followingexample.

containing 1.7 mols of monomethyl adipate and 0.721 mol oftrimethylamine, the remainder. of

the solution -:being methanol) was circulated through a plate-type cellof the type illustrated in the-drawing, containing two platinum cathodes(2 x 4 centimeters) and a platinum anode having an area of 16 squarecentimeters. The circulation was continued for 1.5 hours at a currentdensity of 0.875 ampere per square centimeter at a temperature of 30 C.The voltage gradient was 32.0 to 51.5, an increase in voltage beingcaused by the buildingup of dimethyl sebacate in the system during theelectrolysis. The resulting product was removedfrom the apparatus andwas found to contain 1.192 mols of monomethyl adipate and 0.225 mol ofdimethyl sebacate. The current efficiency was 57.6%. When monoethyladipate is used in place of monomethyl-adipate a considerably loweryield of diethyl sebacate is obtained.

It is to be understood that the foregoing example is illustrative onlyand that numerous em-- bodiments of the invention will occur to thosewho are skilled in the art. For example, when water is employed in placeof methanol as the reaction medium the diester may be withdrawn from thereaction products as an oily layer. Moreover, it is to be understoodthat, in the embodiments of the invention illustrated herein. the

trimethylamine is converted to a salt byreaction.

with the monoester of the dibasic acid, hence the trimethylamine may beintroduced in the form of such a salt, or in any other form which, bysimple interchange or otherwise, yields such a salt. Furthermore, it isto be recognized that the rate of separation of diester from theelectrolysis product obtained in the practice of the invention isprimarily an economic factor, since the current efliciency decreaseswith increased conversion and increased resistance of the electrolysismixture. The dialkyl sebacates which are obtainable in accordance withthe present invention are widely useful as plasticizers and areespecially valuable as intermediates for the preparation of sebacic acidand nylon intermediates which may be obtained therefrom.

Since many embodiments of the invention may be made without departingfrom the spirit and scope thereof I do not limit myself except as setforth in the following claims.

I claim:

. 1. A process for preparing dialkyl esters of dicarboxylic acids of theempirical formula in which n is any integer; which comprises sub-Jecting a methanol solution of a monoalkyi ester 6 anode current densityof about 0.2 to 1.0 ampere per square centimeter.

4. A process for preparing dialkyl esters of dicarboxylic acids of theempirical formula (CHzladCOOHln, in which '11. is any integer, whichcomprises subjecting to electrolysis a methanol solution of monoalkyiester of a dicarboxylic acid of the empirical formula (CHDMCOOHM, saidmonoalkyl ester being partially neutralized with trimethylamine wherebyit contains 0.3 to 0.8 mol of trimethylamine monoester salt per mol ofmonoester in the neutralized plus unneu- 'tralized form, at an anodecurrent density of about 0.2 to 1.0 ampere per square centimeter and atemperature of 0 to 55 C., and thereafter separating the resultantdialkyl ester of of a dibasic acid of the empirical formula(CI-IQMCOOHH, and a trimethylamine salt of the said ester, toelectrolysis and thereafter separating the resultant dialkyl ester of adicarboxylic acid of the empirical formula from the electrolysisproduct.

2. A process for preparing dialkyl esters of dicarboxylic acids of theempirical formula in which n is any integer, which comprises sub-'Jectin'g to electrolysis a methanol solution of monoalkyl ester of adicarboxylic acid of the empirical formula (CHz)n(COOH) 2, said mono.-alkyl ester being partially neutralized with trimethylamine whereby itcontains 0.1 to 0.9 mol of trimethylamine monoester salt per mol ofmonoester in the neutralized plus unneutralized form, and thereafterseparating the resultant dialkyl ester of a dicarboxylic acid of theempirical formula (CI-12):;(COOH): from the a dicarboxylic acid of theempirical formula (CH2)2(COOH)2 from the electrolysis product.

5. A process for preparing dialkyl esters of dicarboxylic acids of theempirical formula (CH2)2n(CQQH)2, in which n is any integer, whichcomprises subjecting to electrolysis a methanol solution of monoalkylester of a dicarboxylic acid of the empirical formula (CH2)n(COOH)2,said monoalkyl ester being partially neutralized with trimethyiaminewhereby it contains 0.3 to 0.8 mol of trimethylamine monoester salt permol of monoester 'in the neutralized plus unneutralized form, at ananode current density of about 0.2 to 1.0 ampere per square centimeterand a temperature of. 20

' to 45 .C. and thereafter separating the resultant dialkyl ester of adicarboxylic acid of the empirical formula (CHMMCOOH): from theelectrolysis product. I

6. The process of claim 5 in which the said monoallwl ester is amonomethyl ester.

'7. The process of claim 5 in which the said monoalkyl ester ismonomethyl adipate.

8. The process of claim 5, in which the said monoaikyl ester ismonoethyl adipate.

WILLIAM F. GRESHAM.

REFERENCES CITED The following references are of record in the file ofthis Patent:

Transactions of the Electrochemical Society.

